Showing X3D color per face?
related, same thing happens with .ply files: use the Vertex Color layer: Import PLY doesn&#39;t display vertex colors 

In Blender there is no color per polygon, but you can set all vertices of individual polygons to the same color. When vertices are shared by multiple Polygons their vertex_color information is not.

If the .x3d is formatted correctly, then Blender will make the appropriate vertex_color map (tested! for instance with this file).